Study the epidemiology of low birth weight and pre-term delivery using the outcome of several successive pregnancies in a large population of women. Since the mother's identification number is recorded on the birth certificates, it is possible to retrieve medical information about consecutive births to the same mother. This system of data collection was begun in 1967, data is now available over a ten year span. This time period will provide a sufficient number of large sibships in order to examine the importance of familial tendencies for pregnancy outcome.